DESCRIPTION: Salmonella typhimurium must periodically endure acid stress during pathogenesis and in the natural environment. Our laboratory has identified a unique acid-inducible survival response that defends against extreme acidic environments. This acid tolerance response (ATR) consists of two clearly defined stages. First, a series of emergency pH homeostasis systems are induced by mild acid and, second, a set of 51 acid shock proteins (ASPs) are induced in response to acid shock. Both stages are crucial for the cell to survive extreme acid stress. The following insights have further defined the ATR and form the basis of the proposed studies. (1) One of the inducible pH homeostasis systems was identified as cadBA encoding lysine decarboxylase (cadA) and a lysine/cadaverine antiporter (cadB). (2) The alternative sigma factor ss and the iron regulatory protein Fur are required for acid tolerance. (3) ss and Fur each regulate separate subsets of ASPs. (4) Acid regulation by Fur is iron-independent. (5) ss is itself an acid shock protein regulated by one of a new family of two component regulators identified as the mouse virulence gene MviA. (6) MviA controls the proteolytic turnover of ss in response to acid shock Specific aims are designed to examine the nature regulation of emergency pH homeostasis systems; identify which among the 14 ss-dependent and Fur-dependent ASPs are critical for acid tolerance; characterize the MviA signal transduction pathway that controls the acid shock induction of ss and define the iron-independent control of acid tolerance by Fur. Strategies to determine how the critical ASPs protect macromolecules have been developed. Finally, mutants defective in emergency pH homeostasis or that are missing key ASPs will be tested through collaborative studies for effects on mouse virulence and in vivo induction in macrophages. This proposal presents a comprehensive research plan designed to reveal details of Salmonella's response to acid. On a higher level, the results will address fundamental questions of global control and general strategies microorganisms use to survive harsh environments.